defmodule Encryptor do
  def rot13(string) do
    string
    |> String.to_char_list
    |> Enum.map(fn ch -> case ch do
         ch when (ch >= ?a) and (ch <= ?z) -> ?a + rem(ch - ?a + 13, 26)
         ch when (ch >= ?A) and (ch <= ?Z) -> ?A + rem(ch - ?A + 13, 26)
         _ -> ch
       end
    end)
    |> to_string
  end
end